olie05 Posted March 2, 2005 Share Posted March 2, 2005 I did a heel toe downshift through a corner the other day and the back tires chirped a little as I was letting out the clutch. Needless to say, I need to work on my heel toe-ing but this got me thinking... IF the engine has the ability to slow the car down when you lift off the throttle, there has to be some kind of force. Then I thought, what if I had a bigger engine in my car? would the force be less, or would it be more? Is there any way of putting the amount of braking force the engine can exert into a number? -Oliver Quote Link to comment Share on other sites More sharing options...
2126 Posted March 2, 2005 Share Posted March 2, 2005 I quess my first question would be....why? However, if you decide to calculate it out, consider the variable. Like rear tire diameter, diff ratio, tranny ratios, engine compression, car weight, aero package items, straight line or during cornering, etc., etc. As you may have already imagined there are many variables to consider. Your question is interesting, but aren't the brakes the responsible party for slowing the car down? Just curious about why you would want to know this information!!? Quote Link to comment Share on other sites More sharing options...
olie05 Posted March 2, 2005 Author Share Posted March 2, 2005 well... If you REALLY want to know, this is the kind of stuff I think of when I stare up at the ceiling here at school... I thought of getting a rough calculation... Since the weight of the car can be found easily, I could do a run from a measured distance and note my speed on the speedometer at the beginning and at the end of the run, and the run would be something like me starting off at 6,000 rpm in 3rd or 2nd gear, and just lifting off the throttle. Notice that I said Rough, since the speedometer (especially mine) is extremely inaccurate... Then I would find out how much force is exerted to reduce the speed of a mass... Then I could do the same thing for acceleration and compare the two... -Oliver Quote Link to comment Share on other sites More sharing options...
2126 Posted March 2, 2005 Share Posted March 2, 2005 Again......just out of curiousity, for what purpose would this information be useful for? Do you have a specific goal in sight? Do you plan to equate this info into some type of performance related issue? I credit your imagination for sure! P.S. So what substance prompted these thoughts? Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted March 2, 2005 Share Posted March 2, 2005 You can still find out. During an engine break, the straight-line mementum of the entire car is partially transferred to angular momentum of certain rotating parts(in other words, car speed goes down while engine speed goes up) "Straight-line momentum loss" = "Net increase in angular momentum of all rotating parts" + "Powertrain loss" There are three ways to increase the "breaking power" of a down shift. 1.Increase the rotating speed of engine after down shift(you accomplish this by using gear ratios that are widely apart, not practical) 2.Increase the moment of inertia of some part of the powertrain(not practical) 3.Increase powertrain loss (not practical) The only way to practically "increase"(relatively speaking ) engine breaking power is to reduce weight of the vehicle. Quote Link to comment Share on other sites More sharing options...
2126 Posted March 2, 2005 Share Posted March 2, 2005 Since I'm a curious fellow TheNeedForZ, please enlighten me on how engine RPM increases as the speed of a vehicle decreases! Some how I get the feeling you have the right ideas in your advise but I don't think your explaining them with technical correctness. Example: "The only way to practically "increase" (relatively speaking) engine braking power is to reduce the weight of the vehicle." How about inreasing the compression of the engine (practicle), or how about changing the ratio in the differential (practicle), or changing tranny gearing (practicle)? Also, I'm still curious about the value of obtaining such information, especially since the brakes are intended for slowing a car down not the engine. Just curious!!! Quote Link to comment Share on other sites More sharing options...
TheNeedForZ Posted March 2, 2005 Share Posted March 2, 2005 ..... I went to read the original question again, I realized I was answering a question that he DID NOT ask. I though he was asking about engine brake(the kind when you down shift) He was actually asking about the natural speed loss when throttle is not applied. so THAT's how you can spend 15 minutes for nothing. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 3, 2005 Share Posted March 3, 2005 You guys have obviously never kick started a old Harley. Or ridden an old dirt bike with a compression brake. Compression ratio. The engine is still pumping and compressing air. It is just getting the work from dragging the rear wheels instead of exploding gas. Higher compression, bigger displacement means more compression loss. Quote Link to comment Share on other sites More sharing options...
olie05 Posted March 3, 2005 Author Share Posted March 3, 2005 Ok, apparently I didn't make myself clear... 2126 - I already answered your question, and seeing as how this is the miscelaneous tech forum, why not have a miscelaneous technical topic? What a thought! There really is no purpose other than to examine the technical side of something that happens in road racing and autocrossing (and sometimes daily driving). and no... I'm not on any *substances* lol... Pop N Wood - It sounds like what you're saying is that the larger the engine, the more resistance to rotating it will have... correct? -Oliver Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted March 3, 2005 Share Posted March 3, 2005 I know a higher compression ratio makes it much harder to crank an engine over. Thus high torque starters with hipo engines. combine that idea with a buddy who raised the compression in his Harley to to point where he could hardly kick it over and I am reasonable certain that high CR make them harder to turn. If you believe that, then it would just make sense that high compression combined with a larger displacement would be even worse. The larger displacement is speculation on my part. Quote Link to comment Share on other sites More sharing options...
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